The Man Who Was Thursday by G. K. Chesterton

Gabriel Syme, a poet turned detective, infiltrates an anarchist council where each member is named after a day of the week. As he tries to expose the mysterious leader “Sunday,” he uncovers layers of deception and existential mystery. The plot evolves from spy thriller to philosophical allegory, questioning identity, order, and chaos. Surreal, witty, and profound, it shifts genre and tone constantly. It's both a detective story and a metaphysical puzzle.